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This invention relates to methods for sensing sound waves, and in particular, using optical means to detect sound waves through certain corresponding changes in the optical properties of air or other optically transparent or semitransparent medium through which the sound propagates.
Current methods for sensing sound waves typically involves the sound waves impinging on a diaphragm or other mechanical surface, and then using an electrical, or in some cases optical, means to detect the movement of said diaphragm or other mechanical surface. Such traditional microphones typically need to be located close to the source of the sound waves so that the diaphragm moves according to the sound waves to be sensed with adequate fidelity and signal strength, and so that the effects of noise and other undesired sounds is minimized. Locating the microphone close to the source of the sound can be problematic, for example, when the microphone is being used to monitor sound waves from a person who needs to be somewhat mobile (for example, a performer on a stage) or does not have a hand free to hold the microphone. Also, traditional microphones typically obstruct the view of a performer""s face. And often microphones need to be hidden from view, for example for motion picture actors and people on television. Or surveillance and security applications may require that the microphone be located a distance from the sound source. Failings with traditional microphones themselves are that they: require cables or other transmission means to relay their sensed sounds to a receiving device, require careful mechanical design and assembly to provide good performance, due to their mechanical nature they have a limited range of frequencies to which they are sensitive, and are often fragile. Also, due to environmental factors, such as electromagnetic fields, pressure, and temperature, or the presence of corrosive or combustible gases, it may be difficult to get acceptable performance from such traditional microphones. And the connectors and cables of traditional microphones are also expensive, subject to wear and damage, and are a source of electromagnetic field induced and electromechanical contact noise. Finally, high quality microphones are large, and as is often seen at press conferences, mounting larger numbers of them on a podium can be a problem.
Prior art has attempted to solve these limitations in a variety of ways.
Wireless microphones are widely used by business meeting presenters, stage performers and other actors. Typically a small battery-operated radio-frequency, infrared light or ultrasonic sound transmitter unit (about the size of a pager) is clipped to the presenter""s belt, and a cable leads from this transmitter to a small microphone, which is often then clipped to the presenter""s clothing, as close to their mouth as is convenient. The transmitter transmits the presenter""s audio to a receiving unit typically located within 2 to 30 meters, and this receiving unit is then connected to an audio amplifier, a recording device or other equipment. There are many shortcomings of this method. For example, the transmitter""s battery can fail at an inopportune time. Radio frequency or other types of interference from other equipment or transmitters can disrupt or degrade the signal. The requirements of routing the microphone cable through one""s clothing, finding a place to mount the transmitter, and mounting the microphone close to the presenter""s mouth are often a problem requiring undesirable trade-offs of audio quality versus convenience. For higher audio quality, larger microphones must be used and positioned in front of the performer""s mouth (as is often seen by singers at concerts), and these obstruct the audience""s view of the performer""s face. Also, each presenter must have a transmitter; and this can be costly, and requires coordination of the radio frequencies used. Alternatively, sharing a unit is difficult as it is awkward to quickly transfer it to another person. Finally, the whole system of microphone, battery, transmitter and receiver is time-consuming to set-up, trouble-shoot and transport.
High-quality microphones are often mounted on long poles. Such boom microphones are often used for television programs and motion pictures. These can be mounted on wheeled dollies; in which case they are they are large, and have heavy counterweights so that the booms can be up to 5 to 10 meters in length. Or the boom microphones can be hand-held, in which case the boom length is typically quite limited, for example to 2 or 3 meters. In any case, such boom microphones typically require a full-time operator to ensure that the microphone itself is as close to the (moving) performer as possible, while staying out of the field of view of the camera (which can often change to a wider view, requiring the microphone to first be moved). Also, the maximum distance from the actor to the microphone is limited by the audio quality required (background noise and reduced frequency response are a problem at more than a meter or two).
U.S. Pat. No. 3,633,705 to Teder describes a microphone with a tubular housing to aid in rejecting unwanted noise. Such microphones are large, and still need to be close to the sound source, are effective for only some frequencies and directions of noise and require careful mechanical assembly.
A microphone mounted near the focus of a paraboloid-shaped plastic reflector is often used to increase the directionality of a microphone, so that such microphones can be located a distance from the sound source, as in U.S. Pat. No 3,895,188 to Ingraham. Due to the nature of such reflected sound waves, such systems have a poor frequency response characteristic, are often too sensitive to wind, handling and other noise sources, pick-up undesired sounds behind the intended sound source, and require trial-and-error focussing adjustments according to the distance to the sound source.
Sensing sound waves and other vibrations (such as those from rotating machinery) through optical means is the subject of much prior art. A method of sensing vibrations and other very small movements of surfaces is described in U.S. Pat. No. 5,029,023 to Bearden et al. This involves coherent laser lit reflected from the measured surface to be fed back into the laser cavity and measuring the resulting varying light output of the laser. The distance over which such a system operates is limited by the coherence length of the laser light source, which is typically less than a meter. Also, to sense sound would require a reflective diaphragm to be located near the sound source, and for this diaphragm to be precisely aimed to return the laser light. Also, the physical characteristics of the diaphragm affect the fidelity of the sensing of the sound waves. Such complex and high-cost systems are typically more suited to experimental and laboratory use than industrial and commercial applications. U.S. Pat. Nos. 5,202,939 and 5,392,117 to Belleville et al. describe an interferometry-based method for detecting small displacements, such as due to the stress of a structural member which is located at the end of an optical fiber cable. Such systems are suited more to instrumentation applications, and require a fiber optic cable to be run all the way to the sound source.
U.S. Pat. Nos. 5,146,083 and 5,200,610 to Zuckerwar et al., and U.S. Pat. No. 5,262,884 to Buchholz describe systems which use a fiber optic cable to illuminate an optical element, with said optical element being mounted on a flexible membrane or diaphragm which vibrates according to the ambient sound waves. The motion of the optical element relative to the illuminating fiber optic cable affects the amount of light directed back to the same or a second fiber optic cable, and this is sensed at the far end of the fiber optic cable. U.S. Pat. Nos. 3,622,791 to Bernard and U.S. Pat No. 5,247,490 to Goepel use a mirror mounted on a diaphragm, which is located so that it moves according to the ambient sound waves, and the motion of the mirror is detected through interferometry. U.S. Pat. No. 4,479,265 to Muscatell describes a cylindrical microphone with a variety of reflecting surfaces which move according to the ambient sound, and this movement is detected by tracking the Doppler shift, beat frequency with a reference beam, or movement of interference fringes, of laser light reflected from these surfaces. Pat. Nos. 4,071,753 to Fulenwider et al. and U.S. Pat. No. 4,166,932 to Selway describe a microphone in which the alignment of two optical fibers is affected by sound waves impinging on a diaphragm, so that the amount of light transmitted through the optical fibers varies according to the sound waves. U.S. Pat. No. 4,566,135 to Schmidt describes a microphone where a membrane in close proximity to the surface of a transparent material is affected by sound waves, so that the reflection of light from a light source in the transparent material, to the surface below the membrane, and to a light sensor within the transparent material is affected, so that the intensity of the received light varies according to the sound waves. All of these methods require a membrane, diaphragm or other surface which must be close enough to the sound source that it vibrates according to the sound. This results in many of the same problems as locating a traditional microphone.
U.S. Pat. No. 4,979,820 to Shakkottai et al. describes a laser light and interferometry-based method of remotely detecting sound waves due to leaks. This method requires a special retroreflective target with a printed Ronchi grating to be located on the opposite side of the sound source. It also requires precise alignment of the split beams of laser light, and a micrometer adjustment to match the pitch of the interference lines to the Ronchi grating.
Clearly, there is a need for a method of sensing sound waves that overcomes these shortcomings.
The present invention describes several optical methods for sensing sound waves, as these have substantial advantages over the prior art, for many applications.
It is an object of the invention to sense sound waves at a distance from the sound source, and that no equipment, device, surface, diaphragm or membrane need be located at or near the sound source, and only a simple reflecting surface or light source need be located past the sound source.
It is a further object of the invention that it provide great selectivity of the sound source sensed, both in the direction of the sound source relative to the invention, and in the direction of propagation of the sound waves.
It is a final object of the invention to sense sound without dependence on the; careful mechanical design, frequency response, temperature or other environmental or handling requirements or characteristics of a diaphragm, membrane or other sound wave responsive surface.
The present invention overcomes important limitations and requirements of traditional microphones.
In one embodiment the present invention, light with a wide range of wavelengthsxe2x80x94for example, ambient light or a beam of light generated for this purposexe2x80x94illuminates means for detecting the optical characteristics of the light after it has passed through the sound waves to be sensed, such means for detecting the optical characteristics of the light being referred to as an xe2x80x9coptical receiverxe2x80x9d herein. Through the use of a diffraction grating, interference filter or other means, the optical receiver in this embodiment is designed to be sensitive only to specific wavelengths of light, those being the wavelengths which are highly absorbed by a specific constituent of air.
In another embodiment of the present invention, the light source emits specific wavelengths, those being the wavelengths absorbed by a specific constituent of air. This embodiment eliminates or simplifies the requirement for optical filtering within the optical receiver.
In another embodiment of the present invention, more than one optical receiver is used to; track variations in the output of the light source to reduce the noise which would be caused by such variations, determine the direction of propagation of the sound waves, provide increased or decreased sensitivity to particular wavelengths of sound, or other purpose.
In another embodiment of the present invention, the narrow band light source emits coherent light, and this light is split into a first and second beam. The first beam is reflected back to the optical receiver, and the second beam passes through the sound waves, after which it is recombined with the first beam within the optical receiver.
In all cases, a path of light between the light source and the optical receiver passes through the sound waves to be detected.
Sound waves are alternately compressed and rarefied air (or whatever medium through which the sound waves are propagating). That is, they are propagating waves of air which are at periodically slightly more, and later slightly less, pressure (and density) than typical atmospheric air. The longer-term atmospheric air pressure depends on the altitude, weather conditions and other factors, but is typically 97,000 to 106,000 N/m2 (the units are newtons per square meter, and these are also referred to as Pascals, which are abbreviated as Pa) near sea level. In air, sound waves propagate somewhat spherically from the sound source at the speed of sound, which is approximately 344 m/s at 21xc2x0 C. and typical atmospheric pressures. That is, sound waves continuously change air""s density. For example, a 1,000 Hz pure tone will sinusoidally increase and decrease the density or the air 1,000 times per second. Similarly, the density of each of the constituents of the air (such as nitrogen, oxygen, carbon dioxide and water vapor) will also sinusoidally increase and decrease 1,000 times per second.
The constituents of air, for example, each have a distinctive spectrum of wavelengths of light which the constituents absorb, due to the resonance and other characteristics of the constituent""s molecular bonds, and other factors. For example, carbon dioxide gas highly absorbs (and therefore attenuates) light at wavelengths of 2.69 xcexcm, 2.76 xcexcm and 4.25 xcexcm, and water vapor absorbs light at wavelengths of 2.66 xcexcm and 2.73 xcexcm. Identifying and quantifying substances through measuring these absorptions is the basis for the field of atomic absorption spectrophotometry, which is also referred to as optical spectroscopy. The amount of attenuation (this is called the absorbance) of a band of wavelengths of light due to absorption by a particular constituent of air is proportional to the density of that constituent. Since sound waves affect the density of air (and therefore of its constituents), by detecting the resulting changing attenuation of light, sound waves passing through the path of the light can be detected.
There is relatively little change in the density of the air, as averaged along a path in the direction of propagation of a sound wave (that is, longitudinally), since the total number of air molecules along the path remains mostly constant (except for the small amount of movement past the ends of the path). However, for a light path that is transverse to (that is, across) the direction of propagation of the sound waves and which passes near the sound source, the increase and decrease in the density of air due to the sound waves can be calculated, and shown to be a measurable amount.
Further, to better detect sounds from a person or animal at a distance, the changing intensity of a wavelength of light which is highly absorbed by carbon dioxide or water vapor could be monitored, since these constituents of air have much higher concentrations in exhaled air and therefore near the mouth of people and animals, and much lower concentration (and therefore attenuation) elsewhere along the light path.
The optical receiver may employ a photodetector to directly measure the changes in the intensity of the received light, or another method of measuring the changing optical characteristics of the light beam passing through the sound waves, such as measuring the changing density of a selected constituent of the air by modulating the light beam and measuring the resulting heating of the constituent through acoustic monitoring.
Alternatively, the greater the density of air (for example, due to sound waves), the slower the velocity of propagation of light. Therefore, a light beam which passes transversely through sound waves will have a varying velocity of propagation of light, according to the sound waves. This varying velocity of propagation of light, will result in a varying phase shift of the light beam, and this can be detected using interferometric means, which are well known.